1. Field of the Invention
The present invention relates to a resonant frequency characteristic tag, which is readily capable of dielectric breakdown of an electrically insulating thin film of dielectric in a high output electric field to short-circuit electrodes on the opposite sides of the dielectric to destroy a preliminarily provided resonant frequency characteristic, and a method of manufacturing the same.
2. Description of the Prior Art
In department stores, supermarkets and general retail shops, various commodity projection means are adopted in order to safeguard commodities against shop-lifting or like theft. Where an electric sensor is used, the commodities are provided with tags each having a resonant frequency circuit in order that the presence of commodity in a control area can be confirmed electronically. More specifically, when a tag is present in an area covered by the sensor and is resonant to a waveform at a certain frequency transmitted from the sensor, the sensor generates a certain alarm sound. Further, in order that such a resonant frequency characteristic tag does not generate any alarm sound in spite of its presence in the coverage area after the purchase of the pertinent commodity has been confirmed, the tag is constructed such that by application of a high output electric field to it a dielectric in its capacitor section is destroyed to short-circuit its capacitor electrodes to each other, thus destroying its resonant frequency characteristic.
The capacitor section of such a tag is fabricated by using a tool in such a manner that its constituent material is squeezed mechanically in the thickness direction to a very thin form so that application of a high output electric field to it will cause electric breakdown of its dielectric thin layer to short-circuit its electrodes to each other.
In order to be able to supply as inexpensive resonant frequency characteristic tags as possible to the marker, the cost of manufacture should be as low as possible. In the aspect of electric characteristics, to obtain a resonant characteristic necessary to the resonant frequency circuit, the material of the tag should be such that it is possible to obtain the intended size and allowance very accurately. To meet such theoretical requirement, a substrate which is used in the prior art tag for providing the resonant frequency is fabricated by applying an aluminum foil having a thickness ranging from 0.05 to 0.009 mm to each side surface of a polyethylene film having a thickness of 0.025 mm and with a thickness allowance of .+-.5%. The aluminum foil is applied by an extrusion process in case of low density polyethylene. In case of high density polyethylene, a thermal press process is used for the application. However, with the substrate that is prepared in the above ways, it is difficult to attain the aim of the invention, i.e., to cause dielectric breakdown of the electrically insulating thin film to short-circuit the two electrodes to each other.
In the method of mechanically squeezing the tag material using a tool, it is difficult to control the working conditions such as to obtain the thickness to a range less than 0.002 mm, in which the dielectric breakdown of the dielectric is possible, due to such causes as thickness fluctuations of the material and difference in working ranges in manufacture. Therefore, it is liable that the dielectric is completely ruptured during the manufacture so that short-circuit of the electrodes can no longer be obtained, or if the urging pressure or like working condition is insufficient, the resultant dielectric film has a thickness too large to cause the dielectric breakdown.